Gridspecs (for MIR)

A gridspec describes spatial grids in the form of a dict.

Warning

The gridspec format is not finalised yet and may change in future releases. Area specification is currently supported only as a provisional feature (subject to change or removal) and may emit deprecation warnings.

The gridspecs supported when regridding with the MIR backend are summarised below:

Gaussian grids

Gaussian grids are a family of global grids defined by the number of constant-latitude lines (parallels) between the pole and equator. Each parallel’s latitude is at Gaussian quadrature points so integrals can be calculated over the sphere. With increasing resolution these parallels approach but do not match the poles and equator. Longitudinally there are two descriptions: regular with constant number of points per parallel, and reduced with varying number of points per parallel, increasingly higher near the equator so as to approach a constant local horizontal resolution over the globe.

There are three patterns for grid, case-insensitive:

OXXX
NXXX
FXXX

where XXX is known as the Gaussian number, and represents the number of latitude lines between the pole and equator. For details about these grids, see here. The regular Gaussian grid, [Ff]XXX, has a constant number of points per parallel, while the reduced Gaussian grid, [NnOo]XXX, has a varying number of points per parallel.

There are a few pre-defined reduced Gaussian grids (eg. N160, N320, N640) which have a pre-defined number of points per parallel. The octahedral Gaussian grid ([Oo]XXX) has a specific number of points per parallel starting from 20 near the poles and increasing 4 points per parallel towards the equator, so it is defined for any Gaussian number.

Examples:

{"grid": "O320"}
{"grid": "o320"}
{"grid": "N320"}
{"grid": "n320"}
{"grid": "F320"}
{"grid": "f320"}

Regular latitude-longitude grids

The grid format is:

[DLON, DLAT]

where DLON and DLAT are the increments in degrees in longitudes and latitudes, respectively. This grid is global and includes the origin (0°, 0°), and the ECMWF’s default scan mode: first point at North-West with longitude the fastest index from West to East.

Example:

{"grid": [1, 1]}
{"grid": [0.1, 0.1]}

HEALPix grids

The grid is case-insensitive, in the format:

HXXX
HRXXX
HNXXX

The HEALPix grid is a global, hierarchical equal area isolatitude pixelisation of the sphere, where XXX is the HEALPix Nside representing the number of points per pixel side. There are 12 base pixels, so the total number of points is \(12 N_\mathrm{side}^2\). The points can be in ring (default) or nested order, indicated by the second letter or order (default ring); if the second letter is [Nn] or order is nested, the points order is nested. For details about this grid, see here.

Examples (the last two are in nested order):

{"grid": "Hr512"}
{"grid": "h512"}
{"grid": "H512", "order": "ring"}
{"grid": "Hn512"}
{"grid": "h512", "order": "nested"}

ORCA grids

These grids are designed for global ocean coverage, and are associated with the NEMO model. The grid format is case-insensitive, in the formats:

ORCAXXX
ORCAXXX_[FTUVW]
eORCAYYY
eORCAYYY_[FTUVW]

where the first letter stands for extended indicating coverage closer to the South pole. T, U, V or W define the point location respective to the supporting mesh elements, respectively in cell-centred, vertex and u/v edges point arrangements. The default arrangement is cell-centred or F.

Horizontal resolution numbers XXX and YYY are in increasing order 2 (XXX only), 1, 025, 12, approximately corresponding to resolution in degrees 2°, 1°, 0.25° and 1/12°.

Examples:

{"grid": "eORCA025_T"}
{"grid": "ORCA1"}